The present invention relates to a toner density control device for an electrophotographic copier, facsimile apparatus, a laser printer or similar image forming apparatus of the type forming an image by depositing a toner on an electrostatic latent image.
Generally, an image forming apparatus of the type described uses a two-component developer which is a mixture of toner particles and carrier particles. The density of an image decreases as the toner content of the developer, i.e., the toner density decreases. It has been customary to form a test pattern on an image carrier in the form of a photoconductive element between images, to sense the image density of the test pattern, and feed a supplementary amount of toner automatically to the developer such that the sensed image pattern remains constant. This kind of implementation, however, has a drawback that once the image density is lowered due to the changes in the charge potential deposited on the photoconductive element, the amount of exposure, the characteristics of the developer and so forth due to aging, an excessive amount of toner is fed to bring about various problems such as excessively high image density, fog, and smear.
Japanese Patent Laid-Open Publication (Kokai) No. 57-13667 discloses a toner density control device which is elaborated to eliminate the above-discussed drawback. Specifically, the device disclosed in this Laid-Open Publication includes image density sensing means for optically sensing the density of a test pattern which is produced by a toner deposited on an electrostatic latent image, and toner density sensing means for sensing the density of a toner which constitutes a developer together with a carrier. The level for sensing the toner density is variable on the basis of the image density which is sensed by the image density sensing means. Such a scheme, however, cannot be implemented without resorting to complicated control. Furthermore, when images each having a substantial area are formed, the distance between test patterns each intervening between the nearby images and, therefore, the interval between toner supply control timings is increased resulting in a difference in density being developed between the leading end and the trailing end of such a large image.